The "Southern Question" is indeed one of the longstanding dilemmas of Italian Economic History. Research into this topic is not as florid as you might initially think: for much of Italy's unitary history the "Southern Question" was perceived as a principally political topic, and it's only in last ten or so years that the "Question" has received steady attention from a growing cadre of economic historians, especially from those who are themselves located in the universities of the Italian South. Paolo Malanima at the University of Catanzaro, Emanuele Felice at the University of Chieti, and Claudia Sunna at the University of the Salento, are just some of the economic historians whose recent books and papers have been well received by their academic peers both in Italy and abroad, igniting new interest in Italy's lopsided economic development.

The present academic consensus is that at the time of unification, the North of Italy was slightly more economically developed than the South of Italy. However, the social and institutional apparatus in the north was better positioned to take advantage of imminent industrialization process that would take place after unity, quickly creating a perceptible gap in development between the two halves of the country. By the turn of the 20th century, a political discourse had already emerged around a "Southern Question" which has not existed at the time of unification (the terminology of "Question" itself seems to have been coined by the influential politician Giustino Fortunato, who served as member of parliament for Melfi between 1880 and 1900).

Of course there is a lot of nuance tied to the study of historic inequality, especially in a late industrializer like Italy. Unfortunately, there are many difficulties tied to creating precisely assessments economic development in Italy immediately before and after unification. While scholars (notably Vera Negri-Zamagni at the University of Bologna) have attempted to quantify economic development by looking over what little data is available with a careful and critical eye, there are still many possible pitfalls: one of many examples is the fact that as late as 1881, southern women who practiced sewing and weaving at home were classified by local censors as “Textile Workers.” When census norms changed and no longer considered work done in the home as "Employment," this caused a precipitous increase in unemployed women to appear in data coming from the South, even though real economic activity hadn't changed much at all.

One stand-in for institutional and social development that researchers have found good to work with is literacy and primary education. In this measure, the pre-unitary Southern Italian Kingdom suffers from a lack of public education policy, entirely reliant as it was on the clergy and their parochial schools. As a result, at the moment of unification in 1861 90% of southern Italians were illiterate, and only 20% of children were able or willing to enroll in primary education. This stands in stark contrast with Piedmont and Liguria, the most significant territories of the Kingdom of Piedmont-Sardinia (the polity which would go on to unify Italy) where in 1861 literacy stood at 53%, and, 93% children were enrolled in primary education thanks to universal education policies. The estimates (postulated by the aforementioned Negri-Zamagni in "La situazione economica e sociale nel meridione negli anni dell’unificazione: Una rivisitazione” published in 2013) certainly can have some issues, as in both north and south measures of literacy were inconsistent. However, it this disparity is corroborated by other indicators: in 1862 (immediately after unity) 6.1 letters per inhabitant were mailed in the northern region of Piedmont, while only 1.6 letters per inhabitant were mailed in the former territories of the Kingdom of Two Sicilies.

The mailing of letters is also reflected in the disparity between professionalized industries in north and south, like banking and insurance, who would be making most use of postal correspondence. Between 1827 and 1848 (thus in the pre-unity period) of the 53 banks chartered all over Italy not one was founded in the Kingdom of the two Sicilies. The south would have to wait till 1850 for its only two commercial banks to be founded (the Bank of Naples and the Bank of Sicily). While we have no fair pre-unity estimates for insurance companies, in the post unity period (in 1865) of the 88 insurance companies registered in Italy only 21 were based in the South, against the 67 in the North. And beyond banking and insurance, in 1865 of the 208 joint-stock industrial corporations in existence in Italy, only 17 were registered in the South; a serious marker of lack of development in a country that was just then joining the industrial revolution (my source is, as above, Negri-Zamagni).

So what stopped the South from developing the hallmarks of an industrial economy, like banks, insurers, and most critically industrial corporations?

The consensus is attributable to the lack of legal or institutional framework favoring capital accumulation in the Kingdom of two Sicilies. Indeed, it would seem that the southern monarchy was itself disinterested in favoring industrial growth, preferring to award of letters patent to foreign investors for what few industrial endeavors might take place. Even things like military and naval procurement was outsourced. The railroad, hallmark of the industrial revolution, was only built to connect the King's summer palace to the King's winter palace. Cause and consequence of this lack of initiative was the southern moneyed classes' preference for investing in extensive agriculture, rather than industry.

The attitude in the north, however, was much different. The Kingdom of Piedmont lived a precarious existence sandwiched between the Austrian and French Empires. A finely honed instinct for survival among its political class, in addition to an influx of political dissidents from the Austrian-held parts of Italy (as well as a few thinkers expelled from the South) fomenting political thought, led the Kingdom to do its best to imitate the great powers of Europe in industrial development. Policies weren't always successful or well thought out, but they were successful enough to turn the maritime city of Genoa into a center of naval construction, connecting Genoa to the capital of Turin via a railroad, and even develop plans for a future connection between Turin and Switzerland.

In Austrian-held Italy too, a political and entrepreneurial class based in and around the city of Milan worked to imitate their peers in Austria, Bohemia, and Hungary, in order to foster economic development: milanese entrepreneurs financed railroads, canalization of waterways, and founded a series of technical and vocational schools (a far cry from the southern system, which as mentioned above relied on parochial schools).

It is worth keeping in mind, though, that Italy at the time of unification was still a fundamentally agricultural economy. France, Great Britain, and parts of Austria were much more developed. The gap between Northern Italy and the industrialized regions of Europe would not close until the 1980s.

But why did the gap close in the North and not in the South?

The question is one of the many that Emanuele Felice tries to answer in his concise but ambitious, "Ascesa e Declino." The north was certainly helped by the presence of a great river, the Po, whose tributaries could be turned into canals to water farms and power mills, and which could be navigated by barges, creating a large unified marketplace even before the construction of the railroad. The south had no such hydrographic advantage. The north's cities were also geographically closer to the industrialized regions of Europe, and thus exchanging goods, materials, and ideas was much easier than in the South. The north would also benefit, in the long run, from the two world wars: Haphazard and hurried wartime planning meant that production of wartime materials was focused where there already was productive capacity, thus turning the north's marginal advantage into a much more substantial advantage.

But Felice also identifies policy decisions taken in the post-war era as important factors. In fact, the gap between north and south seemed to be closing in the wake of the second world war, but would begin increasing again following the Italian response to the global economic slowdown of the 1970s: deliberate decisions were made to direct state-sponsored conglomerates as well as government procurement to favor the country's least developed regions. This created a paternalistic and clientelistic relationship between southern entrepreneurship and the government, with entrepreneurs quickly preferring to follow lucrative subcontracts offered by the state and state-sponsored entities, divesting from existing industries that had driven the south's budding economic growth up to that point.

I'm very bad with conclusions, but that's the summary of the leading academic consensus. Feel free to offer any follow ups or additional questions you may have.